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Membrane Structure and Function
Cell Membranes and Cell Transport
Membrane models
• 1895 – Membranes are made of lipids • 1917 – Phospholipids can form membranes • 1925 – Its actually 2 layers - there are proteins in membranes - membranes are coated by proteins on
both sides. Davson-Danielli Model: Cell membrane is made of
phospholipid bilayer sandwiched between two layers of globular proteins, and is 8 nm thick.
Membrane Model
• Plasma membrane - selectively permeable membrane, controls movement of materials into/out of cell
• Current model is called the fluid mosaic model.
• The most abundant lipid in membranes are phospholipids, which are amphipathic.
• Proteins are embedded in the phospholipid bilayer with hydrophilic regions extending out into the aqueous environment.
Evidence for Fluid Nature
• Membranes are held together by hydrophobic interactions (very weak)
• Membrane lipids drift laterally within the membrane. (Flip-flop rare)
• Membrane proteins drift as well, although more slowly.
• There must be fluidity for membrane to function: Solidification changes permeability and enzyme action
• The fluid part of the model is supported by evidence.
• Membranes remain fluid as temperature decreases, until it solidifies,
• variation in temperature of solidification based on fatty acid type and the amount of cholesterol.
• Cholesterol decreases membrane fluidity at moderate temps – reduces phospholipid movement.
• Cholesterol slows down membrane solidification by disrupting regular packing of phospholipids.
Mosaic
• A membrane is a collage of different proteins embedded in the fluid of the lipid bilayer.
• Two major types of membrane proteins:
– Integral proteins go through the hydrophobic core
– Many are transmembrane proteins which completely cross the membrane.
– Hydrophobic regions of proteins consist of non-polar amino acid sections.
Membrane Proteins
• Each membrane has a unique set of membrane proteins which determine most of the specific functions of that protein.
• Integral proteins are often transmembrane with two hydrophilic ends and a hydrophobic middle.
• Peripheral proteins are attached to the surface of the membrane, often attached to integral proteins.
• Proteins attach to the cytoskeleton on the cytoplasmic side and fibers of the ECM on the exterior – for support.
Major Functions of Membrane Proteins
1) Role of Membrane Carbohydrates in Cell-Cell Recognition
– The ability of a cell to identify other cells is based on recognition of membrane carbohydrates.
– Glycolipids ( ) and glycoproteins ( ) attaced to the outside of plasma membrane vary within species, within individuals, and among cell types.
2) Synthesis and Sidedness of Membranes
Membranes have inner and outer faces, related to the composition of the lipid layers, the directional orientation of their proteins, and the attachment of carbohydrates to the exterior surface.
3)Enzymatic Activity
A protein built into the membrane may be an enzyme with its active site exposed to substances in the adjecent solution. In some cases a metabolic pathway forms.
4) Transport – channel proteins and carrier proteins
Selective Permeability
• The plasma membrane permits a regular exchange of nutrients, waste produts, oxygen, and inorganic ions.
• Permeability of the Lipid Bilayer:
• Hydrophobic, nonpolar molecules, such as
hydrocarbons, CO2 and O2 can dissolve in and cross through a membrane.
Differentially Permeable
Transport Proteins
• Ions and polar molecules may move across the plasma membrane with the aid of transport proteins. Hydrophilic passageways through a membrane are provided for specific molecules by channel proteins. (Ex: aquaporins – help H2O cross)
Carrier Protein
• Carrier proteins may physically bind and transport a specific molecule.
Passive Transport
• Passive transport is diffusion of a substance across a membrane with no energy expended.
• Diffusion – the movement of a substance down its concentration gradient due to random thermal motion.
• The diffusion of one solute is unaffected by the concentration gradient of other solutes. The cell does not use energy when substances diffuse across membrane down their concentration gradient, why it is a type of passive transport.
A solution is…
Answer:
• solute + solvent solution
• NaCl + H20 saltwater
Osmosis
• Osmosis is the diffusion of water across a selectively permeable membrane. Water diffuses down its own concentration gradient, which IS AFFECTED by the solute concentration.
• Binding of water molecules to solute particles lowers the proportion of unbound water that is free to cross the membrane.
Tonicity • When there are two
solutions separated by a semi permeable membrane (such as a cell) the water will move across the membrane based on whether the membrane is permeable to the solute(s).
ISOTONIC
• If a cell is placed in an isotonic environment there will be no net movement of water. (Cell won’t gain or lose H2O)
HYPERTONIC
• If the cell is placed in a hypertonic environment the cell will lose water and shrink.
HYPOTONIC • If the cell is placed in a hypotonic environment
the cell will gain water and expand and possibly burst.
• REMEMBER THESE TERMS ARE RELATIVE…
• Cells without a cell wall must either live in an isotonic environment, such as salt water, or bathed in body fluids, or use osmoregulation.
Turgor Pressure
• Cells with a rigid cell walls in hypotonic environments swell putting pressure on the cell wall.
• This pressure is called turgor pressure, and provides mechanical support. Plant cells is isotonic surroundings are flaccid, and the plant will wilt.
• In hypertonic – plasmolysis occurs – plasma membrane pulls away from cell wall.
Isotonic environment
Hypotonic Environment
Hypertonic Environment
Facilitated Diffusion
• Facilitated diffusion is the diffusion of polar molecules and ions across the membrane using transport proteins: channel proteins or carrier proteins.
• Channel proteins: aqua porins, ion channels
• Carrier proteins: Conformational change causes movemnt of binding site, and attached solute across the membrane.
Facilitated Diffusion
Active transport
• Requires spending the cell’s own energy.
• Essential for the cell to maintain an internal concentration of small molecules that differ from environmental concentrations.
• ATP transfers its last P to a carrier protein, causing conformation change and movement of solute into cell.
• The sodium-potassium pump works this way to exchange Na+ and K+ across cell membranes, to create a greater concentration of potassium ions and a lesser concentration of sodium ions within the cell.
• Membrane potential - voltage across the plasma membrane due to unequal distribution of ions. Electrical potential energy – cytoplasm is negatively charged compared to ECF ( charged)
• Proton pump generates membrane potential by transporting H+.
• Cotransport – Coupled transport by a membrane protein – transport of a solute indirectly driven by ATP powered pump that transports another substance against its gradient. As the substance diffuses back through a special cotransporter, the solute is carred across the membrane.
Cotransport
Bulk Transport
• Exocytosis – cell secretes macromolecules by fusing a vesicle with the plasma membrane.
• Endocytosis – a region of the plasma membrane moves inward and pinches off to form a vesicle containing material from outside the cell.
• Phagocytosis - food particle, pseudopodia wrap around the food particle, creating a vacuole and then…
• Pinocytosis – droplets of ECF are taken into the cell.
• Receptor-mediated endocytosis – allows for the intake of specific substances.
Exocytosis
Phagocytosis
Pinocytosis
